Multi-aperture polarization optical systems using beam splitters

ABSTRACT

An optical system includes: a beam splitter system configured to split an input beam into a plurality of output beams including a first output beam, a second output beam, and a third output beam; a first polarizing filter having a first polarization angle and configured to filter the first output beam to produce a first filtered output beam; a second polarizing filter having a second angle of polarization and configured to filter the second output beam to produce a second filtered output beam; and a third polarizing filter having a third angle of polarization and configured to filter the third output beam to produce a third filtered output beam, the first, second, and third angles of polarization being different from one another.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to and the benefit of U.S. Provisional Patent Application No. 63/030,723, filed in the United States Patent and Trademark Office on May 27, 2020, the entire disclosure of which is incorporated by reference herein.

FIELD

Aspects of embodiment of the present disclosure relate to multi-aperture polarization optical systems using beam splitters.

BACKGROUND

A polarizing filter, polarization filter, or polarizer is configured to filter incoming electromagnetic radiation such that it passes portions of the electromagnetic radiation that are aligned with an angle of polarization (or polarization angle) of the polarizing filter and blocks other portions of the radiation having other angles of polarization. As a result, the electromagnetic radiation that passes through the polarized light is polarized in that substantially all of that radiation is aligned with the angle of polarization of the polarizing filter.

SUMMARY

Aspects of embodiments of the present disclosure relate to multi-aperture polarization optical systems using beam splitters. Some aspects of the present disclosure relate to optical systems that include beam splitters configured to split incoming electromagnetic radiation (e.g., light) into multiple beams and passing the separate beams through different polarizing filters having different angles of polarization. Accordingly, such optical systems can be used to concurrently or simultaneously capture multiple polarization images at different angles of polarization from the same viewpoint or along the same optical axis.

According to one embodiment of the present disclosure, an optical system includes: a beam splitter system configured to split an input beam into a plurality of output beams including a first output beam, a second output beam, and a third output beam; a first polarizing filter having a first polarization angle and configured to filter the first output beam to produce a first filtered output beam; a second polarizing filter having a second angle of polarization and configured to filter the second output beam to produce a second filtered output beam; and a third polarizing filter having a third angle of polarization and configured to filter the third output beam to produce a third filtered output beam, the first, second, and third angles of polarization being different from one another.

The optical system may further include a sensor system configured to detect a plurality of polarized intensity images including: a first polarized intensity image of the first filtered output beam; a second polarized intensity image of the second filtered output beam; and a third polarized intensity image of the third filtered output beam.

The optical system may further include a processing circuit and memory, the memory storing instructions that, when executed by the processing circuit, cause the processing circuit to: receive the first polarized intensity image, the second polarized intensity image, and the third polarized intensity image; and compute a polarization signature based on the first polarized intensity image, the second polarized intensity image, the third polarized intensity image, and a plurality of calibration parameters associated with the beam splitter system.

The calibration parameters may be computed based on computing modified Fresnel signal power coefficients in accordance with an index of refraction of the beam splitter system and an angle of the beam splitter system with respect to the input beam.

The calibration parameters may include a modified degree of linear polarization and a modified angle of linear polarization.

The calibration parameters may include a statistical model of the beam splitter system, the statistical model being trained on a dataset collected from images captured through the beam splitter system.

The dataset may include first images corresponding to images captured from reflections of polarized light off a beam splitter of the beam splitter system and second images corresponding to images captured from transmissions of polarized light through the beam splitter of the beam splitter system.

The calibration parameters may include wavelength-dependent transmission and reflection curves.

The processing circuit may be configured to correlate the polarized intensity images and to compute per-pixel polarization values across the correlated polarized intensity images.

The plurality of output beams may further include a fourth output beam.

The optical system may further include: a fourth polarizing filter having a fourth angle of polarization and configured to filter the fourth output beam to produce a fourth filtered output beam, the fourth angle of polarization being different from the first, second, and third angles of polarization.

The optical system may further include: a low pass filter configured to filter the fourth output beam to produce a fourth filtered output beam, and a sensor configured to detect an infrared intensity image of the fourth filtered output beam.

The optical system may further include: a high pass filter configured to filter the fourth output beam to produce a fourth filtered output beam, and a sensor configured to detect an ultraviolet intensity image of the fourth filtered output beam.

According to one embodiment of the present disclosure, a method for computing a polarization signature includes: receiving, by a processor: a first polarized intensity image representing the intensity of a first output beam after passing through a first polarizing filter at a first polarization angle; a second polarized intensity image representing the intensity of a second output beam after passing through a second polarizing filter at a second polarization angle; and a third polarized intensity image representing the intensity of a third output beam after passing through a third polarizing filter at a third polarization angle, the first output beam, the second output beam, and the third output beam being split from an input beam by a beam splitter system; and computing, by the processor, a polarization signature based on the first polarized intensity image, the second polarized intensity image, the third polarized intensity image, and a plurality of calibration parameters associated with the beam splitter system.

The calibration parameters may be computed based on computing modified Fresnel signal power coefficients in accordance with an index of refraction of the beam splitter system and an angle of the beam splitter system with respect to the input beam.

The calibration parameters may include a modified degree of linear polarization and a modified angle of linear polarization.

The calibration parameters may include a statistical model of the beam splitter system, the statistical model being trained on a dataset collected from images captured through the beam splitter system.

The dataset may include first images corresponding to images captured from reflections of polarized light off a beam splitter of the beam splitter system and second images corresponding to images captured from transmissions of polarized light through the beam splitter of the beam splitter system.

The calibration parameters may include wavelength-dependent transmission and reflection curves.

The method may include correlating the polarized intensity images and computing per-pixel polarization values across the correlated polarized intensity images.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention.

FIG. 1 is an abstract depiction of a portion of an optical system according to one embodiment of the present disclosure.

FIG. 2A is a schematic depiction of a portion of an optical system according to one embodiment of the present disclosure having a beam splitter system that includes two beam splitters.

FIG. 2B is a schematic depiction of a portion of an optical system according to one embodiment of the present disclosure having a beam splitter system that includes three beam splitters.

FIG. 3A is an example of a plate beam splitter.

FIG. 3B is an example of a pellicle beam splitter.

FIG. 3C is an example of a cube beam splitter.

FIG. 3D is a schematic diagram of a non-polarizing cube beam splitter.

FIG. 4A is a schematic depiction of an optical system including a prism beam splitter system according to one embodiment of the present disclosure.

FIG. 4B is a schematic depiction of an optical system including a prism beam splitter system according to one embodiment of the present disclosure.

FIG. 5 is one example of a cube beam splitter set up in a controlled laboratory environment with a dielectric coating on the reflecting or beam splitting surface for acquiring empirical data according to one embodiment of the present disclosure.

FIG. 6 is a flowchart depicting a method of computing corrected output images based on received input images and calibration parameters of a beam splitter according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, by way of illustration. As those skilled in the art would recognize, the invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein.

Aspects of embodiments of the present disclosure relate to multi-aperture polarization optical systems using beam splitters. Some aspects of the present disclosure relate to optical systems that include beam splitters configured to split incoming electromagnetic radiation (e.g., light) into multiple beams and passing the separate beams through different polarizing filters having different angles of polarization. Accordingly, such optical systems can be used to concurrently or simultaneously capture multiple polarization images at different angles of polarization from the same viewpoint or along the same optical axis.

Polarization imaging provides significant additional information about a scene that would not otherwise be available using a standard camera system (e.g., a camera system lacking a polarizing filter). As one example, shape from polarization (SfP) provides techniques for using polarization signatures to compute the surface normals (e.g., the directions in which surfaces of objects are facing) using the Fresnel equations. In addition, the surface normals computed from these polarization signatures may also be combined with coarse depth maps (which may be captured using depth sensors such as stereo depth camera systems or time of flight depth cameras such as the Microsoft® Kinect® 2) to capture accurate representations of the shapes of objects in a scene as described, e.g., Kadambi, Achuta, et al. “Polarized 3D: High-quality depth sensing with polarization cues.” Proceedings of the IEEE International Conference on Computer Vision. 2015. The Polarized 3D approach described in Kadambi et al. also describes an ability to resolve azimuthal ambiguity and zenith component estimation using priors from the coarse depth map and knowledge about the material properties of the objects.

The Polarized 3D approach outlined in Kadambi et al. requires multiple images (e.g., three images) of the same scene with differently oriented linear polarizing filters in the optical path of the imager. These multiple images may be used to compute polarization signatures such as Stokes vectors, angle of linear polarization (AOLP or AoP), and degree of linear polarization (DOLP or DoP). Some approaches to computing polarization signatures such as AOLP and DOLP involve the use of three or more polarization images or polarization raw frames corresponding to images captured of a same scene where each polarization image is captured through a polarizing filter set at a different angle (e.g., a first polarization raw frame captured through a polarizing filter set at an angle of 0°, a second polarization raw frame captured through a polarizing filter set at an angle of 60°, and a third polarization raw frame captured through a polarizing filter set at an angle of 120°). Some in some approaches, these multiple polarization images along the same optical path are captured in sequence, with the polarizing filter oriented at different angles in each of the images. However, this approach requires that the imaged scene and the imager remain stationary or otherwise the same between the different captures, and may therefore be inappropriate in dynamic environments, where the objects in the scene, the camera, and or the illumination conditions may change from one image to the next.

Some commercially available sensors, such as the IMX250MZR polarization image sensor available from Sony® Corporation, use a diffractive element-based polarizing filter that is overlaid over the pixel array as part of the sensor fabrication process, where per-pixel polarizing filters having one of four different orientations (e.g., 0°, 45°, 90°, and 135°) are arranged in a pattern similar to a Bayer filter, such that adjacent pixels detect light at different angles of polarization. However, because the differently oriented polarizing filters are adjacent to each other, there is pixel level optical and electrical crosstalk that introduces noise into the independent polarization channels that one derives from the raw data. Even given the fact that the pixels, being global shutter pixels, are typically much larger than nominal rolling shutter pixels this crosstalk is nonnegligible. In addition, the yield hit in manufacturing a sensor of a given resolution is much higher when the polarization channels are sub-sampled much like the chroma and luma channels are subsampled in a traditional sensor with Bayer filter, which may be a reason that the overall cost of the sensor is significantly higher than that of a conventional sensor without the pixel-level polarizing filters (as of 2020, the cost of the IMX250MZR polarization image sensor is more than double the cost of a comparable conventional sensor).

One possible solution for addressing the problems outlined above is to use multiple camera modules arranged in a tightly spaced array, such as a 2×2 array. Each of the four camera modules in a 2×2 array may have its own polarizing filter, each at a different orientation (e.g., 0°, 45°, 90°, and 135°) and the four independent camera modules may be optically isolated from one another to prevent any inter-polarization optical crosstalk. While this solution addresses the problems outlined above with respect to concurrently or simultaneously capturing multiple different polarization images and with respect to crosstalk in the single polarization image sensor such as the Sony® product discussed above, it also introduces a new problem in that the multiple camera modules, though tightly spaced, are located at different viewpoints with respect to the scene, and therefore the independent polarization channels captured by the camera array exhibit parallax shifts between the different channels. The magnitude of the parallax shift is dependent on the object distance and the spacing of the camera modules, and may need to be corrected in real-time or may constrain the operating distance of the camera to distances where the parallax shift is negligible. Nevertheless, even small errors or artifacts that remain after the correction can significantly impact the computation or estimation of the surface normals from the polarization channels. Accordingly, even after spending significant computational resources on correcting the images, there is no guarantee that these errors due to parallax shifts will be completely eliminated.

Accordingly, aspects of embodiments of the present disclosure relate to optical systems that are capable of concurrently or simultaneously capturing multiple polarization images at different angles of polarization along a same optical axis, thereby intrinsically eliminating the parallax offsets. Some aspects of embodiments of the present disclosure are directed to using beam splitters to achieve this end.

FIG. 1 is a schematic depiction of a portion of an optical system according to one embodiment of the present disclosure. In the embodiment shown in FIG. 1, the optical system 1 receives an input beam 10 and supplies the input beam 10 to a beam splitter system 100. The beam splitter system 100 splits the input beam 10 into multiple output beams 20, where the output beams 20 pass through corresponding polarizing filters 200 to generate filtered output beams 30. A sensor system 300 may then detect images corresponding to the intensities of the filtered output beams 30. The images captured by the sensor system 400 may then be supplied to a controller 400, which is configured to compute processed image outputs based on the received images. These processed image outputs may include, for example, polarization signatures (e.g., Stokes vectors, AOLP, and DOLP, as noted above) based on polarization raw frames captured by the sensors 300 that have polarizing filters 200 in their optical paths.

In the embodiment shown in FIG. 1, the beam splitter system 100 produces a first output beam 21, a second output beam 22, and a third output beam 23, which pass through a first polarizing filter 201, a second polarizing filter 202, and a third polarizing filter 303 to produce a first filtered output beam 31, a second filtered output beam 32, and a third filtered output beam 33, respectively. For example, a first sensor 301, a second sensor 302, and a third sensor 303 may be arranged in the optical paths of the first filtered output beam 31, the second filtered output beam 32, and the third filtered output beam 33, respectively. The three polarization raw frames captured by the first sensor 301, second sensor 302, and third sensor 303 are supplied to the controller 400, which is configured to compute processed image outputs such as polarization signatures from the three polarization raw frames.

In various embodiments of the present disclosure, the controller 400 may also control the operation of an imaging system that includes the optical system 1, such as triggering the sensor system 300 to begin capturing images or to perform a reset, controlling a lens system, such as changing a focus distance of the optical system or an aperture of a lens system, and controlling a mechanical shutter system.

While some aspects of a controller 400 may be implemented locally, such as through one or more processing circuits connected directly to the sensor system 300 and physically mounted in a same package or housing as the sensors and the optical system 1, embodiments of the present disclosure are not limited thereto, and some operations performed by the controller 400 may be performed remotely, such as on processing circuits connected to the sensor system 300 via a wired or wireless network connection, either on a local network or on a wide area network (e.g., on a server in a data center). As such, the controller 400 according to various embodiments of the present disclosure may include one or more processing circuits that are local or remote to the optical system 1 and that may be of different types such as a central processing unit (CPU), a graphics processing unit (GPU), a field programmable gate array (FPGA), and/or an application specific integrated circuit (ASIC) such as an image signal processor (ISP), a digital signal processor (DSP), a neural accelerator processor, and the like. The controller 400 further includes one or more memories storing instructions that, when executed by the processing circuits, cause the processing circuits to perform operations in accordance with embodiments of the present disclosure, such as computing the processed image outputs. The one or more memories may also store data including the data received from the sensor system (e.g., captured images), the processed image outputs, and intermediate working data.

Typically, an optical imaging system such as a camera system will include a lens system for focusing light from a scene onto the sensing system. In some embodiments, the lens system is included in front of the beam splitter 100, such that the input beam 10 was formed by the lens system. In some embodiments, the lens system is included behind the beam splitter, such as in the optical paths of the output beams 20 or the filtered output beams 30. In some embodiments, the lens system is located in two or more of these locations (e.g., in front of the beam splitter 100, between the beam splitter 100 and the polarizing filters 200, and/or between the polarizing filters 200 and the sensor system 300). Placing the lens system behind the beam splitter enables the use of separate lenses for each of the separate sensors, but requires performing separate calibrations for each independent combination of lens and sensor for relative centration and other intrinsics to achieve pixel-level correlations between the images captured by the different sensors. However, this approach may also increase flexibility in the design of the optical system. On the other hand, placing a single lens system in front of the beam splitter system 100 simplifies the lens calibration process and reduces costs. Calibration processes according to embodiments of the present disclosure will be described in more detail below.

FIG. 2A is a schematic depiction of a portion of an optical system according to one embodiment of the present disclosure in which the beam splitter system 100 includes two beam splitters 110 and 120. Like reference numerals between FIG. 1 and FIG. 2A refer to like elements and descriptions thereof will not be repeated herein. In the arrangement shown in FIG. 2A, the input beam 10 enters a first beam splitter 110 of the beam splitter system 100. The first beam splitter 110 splits the input beam into the first output beam 21 and an intermediate beam 24. The first output beam 21 passes through the first polarizing filter 201 to produce the first filtered output beam 31, which is detected by the first sensor 301. The intermediate beam 24 proceeds to the second beam splitter 120, which splits the intermediate beam 24 into the second output beam 22 and the third output beam 23, which are filtered by second polarizing filter 202 and third polarizing filter 203, respectively to produce the second filtered output beam 32 and the third filtered output beam 33. The second sensor 302 and third sensor 303 detect images corresponding to the intensities of the second and third filtered output beams 32 and 33

The sensors of the sensor system 300 may be spaced such that the overall path lengths for each beam is substantially the same. Keeping the path lengths of the beams the same may be particularly helpful in controlling aberrations that are a function of the optical path length. In addition, keeping the optical path lengths the same may be important in embodiments where the lens system is in front of the beam splitter system 100, as it allows the lens system to control the focus distances of all sensors 300 of the imager together.

In addition, the beam splitter system 100 according to embodiments of the present disclosure is configured to split the beams into substantially identical beams, e.g., beams having substantially the same power and substantially the same distribution of the power over a spectral band of interest (e.g., the visible portion of the electromagnetic spectrum, a near infrared band, an ultraviolet band, combinations thereof, and the like). For example, in the embodiment shown in FIG. 2A, the first beam splitter 110 may be configured to reflect ⅓ of the input beam 10 and transmit ⅔ of the input beam 10, such that the first output beam 21 has half the intensity of the transmitted beam 24, but, in the ideal case, is otherwise substantially the same as the transmitted beam 24. The second beam splitter 120 may then be configured to reflect ½ of the intermediate beam 24 and transmit ½ of the intermediate beam 24, such that the first output beam 21, the second output beam 22, and the third output beam 23 are, in the ideal case, substantially the same and have substantially the same power (e.g., each representing ⅓ of the input beam 10). While FIG. 2A presents a schematic arrangement where the beam splitter system 100 splits the input beam 10 into three substantially identical beams, embodiments of the present disclosure are not limited thereto and may be configured to split the incident light into more than three beams, such as four or more beams.

FIG. 2B is a schematic depiction of an optical system having a beam splitter system with three beam splitters configured to split an incident beam into four separate beams. In the system shown in FIG. 2B, the first beam splitter 110 splits an input beam 10 into a first intermediate beam 24 and a second intermediate beam 25. The second beam splitter 120 splits the first intermediate beam 24 into a second output beam 22 and a third output beam 23. Likewise, a third beam splitter 122 splits the second intermediate beam into the first output beam 21 and a fourth output beam 26. In the arrangement shown in FIG. 2B, in embodiments where the four output beams 21, 22, 23, and 26 are ideally substantially equal in power, each of the three beam splitters 110, 120, and 122 may be configured to transmit 50% (or ½) of its incoming beam (e.g., input beam 10, first intermediate beam 24, or second intermediate beam 25, respectively). However, as discussed in more detail below, a practical beam splitter may introduce nonlinearities that may be corrected in accordance with aspects of embodiments of the present disclosure.

In the embodiments shown in FIGS. 1 and 2A, a separate polarizing filter is placed in the optical path of each of the output beams 20 to produce filtered output beams 30. For example, in the case of the optical system of FIG. 2B, the four output beams 21, 22, 23, and 26 may each be filtered by corresponding different polarizing filters 200 (e.g., polarizing filters at different angles of polarization, such as 0°, 45°, 90°, and 135°) to produce four filtered output beams. However, embodiments of the present disclosure are not limited thereto, and some of the output beams may be detected by the sensor system 300 without passing through additional optical filters or through other filters. For example, in some embodiments, an output beam (e.g., a fourth output beam) passes through a low-pass filter such that a corresponding sensor (e.g., a fourth sensor) detects infrared light. In some embodiments, an output beam (e.g., a fifth output beam) passes through a high-pass filter, such that a corresponding sensor (e.g., a fifth sensor) detects ultraviolet light.

As noted above, the beam splitter system 100 may include one or more beam splitters (e.g., 110 and 120 in FIG. 2) to split the input beam 10 into the multiple output beams 20. Generally, there are multiple approaches to splitting the incoming rays from the scene to different sensors, including plate beam splitters, cube beam splitters, and pellicles. All of these approaches functionally act as beam splitters, but have their individual advantages and disadvantages.

Plate beam splitters are thin, tilted plates with a beam splitting coating on one side. FIG. 3A is an example of a plate beam splitter 130, that may be used as a beam splitter in the beam splitter system. As shown in FIG. 3A plate beam splitters are tilted plates on which light is incident. Incident light 13 is partly reflected 13-1 on the front surface 131 of the plate and then, after undergoing some refraction 13-2, encounters the rear surface 132 of the plate and undergoes a second partial transmission 13-3 and reflection 13-4 in the direction of the original reflected light 13-1. This results in ghosting 13-5 in the reflected image capture, which can be reduced or prevented by an anti-reflective coating on the rear surface 132. The amount of partial reflection versus partial transmission of the plate beam splitter 130 is controlled based on the characteristics of the front surface 131, such as by coating the front surface 131 with a partially transparent thin coating of metal (e.g., using a physical vapor deposition method) or an applied “polka dot” pattern.

FIG. 3B is an example of a pellicle beam splitter 140. The pellicle approach to beam splitting avoids the ghosting caused by the second partial reflection off the rear surface of the plate beam splitter by using an ultra-thin polymer film 141 stretched over a metal frame 142. Accordingly, the incident light 14 is partially reflected 14-1 and partially transmitted 14-2. This almost entirely eliminates the reflection at the rear surface of the polymer film 141, such that the reflected beam 14-1 does not exhibit noticeable ghosting, but does so at the cost of a very fragile design, as the ultra-thin polymer film 141 is sensitive to vibration and can also easily be damaged (e.g., torn) if the optical system is mishandled (e.g., dropped or bumped).

FIG. 3C is an example of a cube beam splitter. A cube beam splitter 150 uses right angled triangular prisms 151 and 152 to achieve multiple optical paths. The triangular prisms 151 and 152 are joined (e.g., glued) together using an adhesive 153 such as polyester, epoxy, or urethane-based adhesives. The amount of incident light 15 that is reflected 15-1 versus transmitted 15-2 at the interface between the two right-angled triangular prisms 151 and 152 can be controlled by the thickness and other material properties of the adhesive layer 153.

Cube beam splitters provide high quality beam splitting and are simple to mount in an optical system, as they are robust to vibration (as may be experienced in noisy and busy environments such as manufacturing facilities). Cube beam splitters also enable equal path lengths for the reflected and transmitted beams, which has the advantage of controlling aberrations. However, the right angled prisms of a cube beam splitter have multiple surfaces that could potentially create unwanted ghost reflections, notwithstanding the anti-reflective (AR) coatings that may be applied to these other surfaces. In addition, the prisms generally add significant mass to the system, which may be an important trade-off in applications where the size and weight of the imager is an issue.

In some embodiments of the present disclosure, the incoming incident light 10 is passed through a collimator in front of the beam splitter system 100. Collimating the incoming incident light 10 ensures that no aberrations are added to the imaging system and no displacement of the transmitted and reflected beams occur. In embodiments of the present disclosure where the optical design of the imager calls for a focusing light, then there is a spherical aberration component that is a function of the numerical aperture (NA) of the system, index of refraction, and thickness. Nevertheless, the use of a collimator to collimate the incoming incident light does not always ensure that there will be no Angle of Incidence (AOI) at the coating surface of the beam splitter.

Accordingly, some embodiments of the present disclosure use a prism approach to split the incoming light 10 to be supplied to multiple different sensors 300, where each sensor is associated with a separate optical polarizing filter 200.

As noted above, optical systems in accordance with some embodiments of the present disclosure are intended to capture polarization information of an input beam, such as by capturing three or more images of a same scene with polarizing filters set at different angles of linear polarization (e.g., 0°, 60°, and 120°), thereby enabling the calculation of polarization signatures such as Stokes vectors, AOLP, and DOLP.

In order to capture this polarization information, a beam splitter system 100 according to some embodiments of the present disclosure, the output beams 20 (e.g., the transmitted and reflected rays) also preserve the polarization information contained in the input beam 10 (or preserve the state of polarization of the input beam in the transmitted and reflected rays). In such embodiments, the beam splitter system 100 uses non-polarizing beam splitters which can split light at specific, designed reflection/transmission ratios while maintaining the original polarization state of the incident light or input beam 10. FIG. 3D is a schematic diagram of a non-polarizing cube beam splitter 160 having a first prism 161, and second prism 162, and a non-polarizing beam splitter coating 163 disposed therebetween. An input beam 15 enters the first prism 161 and is partially reflected and partially transmitted when it meets the non-polarizing beam splitter coating 163 to produce a reflected beam 16-1 and a transmitted beam 16-2. As shown in FIG. 3D, the polarization state (indicated by the axes showing the plane of incidence having p-like (P) and s-like (S) axes) of the input beam 16 is preserved in the reflected beam 16-1 and the transmitted beam 16-2.

As noted above, the reflection/transmission ratio of the beam splitter coating is controlled to ensure that the three or more output beams 20 of the beam splitter system 100 each have the same intensity (e.g., such that, in the absence of filters such as polarizing filters, each of the sensors 300 would receive the same amount of light). Nevertheless, various commercially-available non-polarizing beam splitters exhibit non-linearities, such as wavelength-dependence, in the transmission of light through the beam splitter. For example, one non-polarizing beam splitter coating offered on products made by Edmund Optics Inc. of Barrington, N.J., transmits light in a range from about 37% at a wavelength of 400 nm, rising to a peak transmission of about 53% at a wavelength of about 500 nm, then gradually dropping to about 45% at 700 nm. Furthermore, the transmission curves differ between p-polarized and s-polarized light, where s-polarized light may start at a transmission of about 40% at 400 nm, rise to 50% at about 500 nm, then gradually fall to about 43% at 700 nm, while the transmission of p-polarized light may start at about 35% at 400 nm, rise to about 55% at 500 nm, then fall to about 47% at 700 nm. In the case of an ideal non-polarizing beam splitter, the percentage of light that is transmitted through the beam splitter should be flat across the electromagnetic spectrum (or at least the portion of interest). However, the reality of the non-linearity of the transmission ratios of non-polarizing beam splitter coatings indicate that a simple design that splits the optical path using such a beam splitter has unintended consequences in that it changes the spectral response and relative signal-to-noise-ratio (SNR) at each sensor. Portions of the light that are not transmitted through the beamsplitter are assumed to be reflected by the beamsplitter, or some portion may be absorbed by the beamsplitter. Accordingly, similar reflection curves may also be measured from beamsplitter devices during a calibration process.

While the dielectric non-polarizing beam splitter coating of the beam splitter ideally preserves the polarization state of the input beam in the reflected and transmitted light, the degree of preservation of polarization state may also include imperfections and non-linearities. In practice, the degree and angle of polarization of light detected by the sensors are going to be modified differently from each other and the image capture by each sensor with its own corresponding linear polarizing filter will have a correspondingly different signal.

In some embodiments, the sensors, stacked with polarizing filters, are placed directly on the sides of the prisms of the beam splitter (e.g., cube beam splitter), in a manner similar to that used by 3-CCD (charge-coupled device) cameras, which used a dichroic (or trichroic) beam splitter prism to split the input beam into separate red, green, and blue beams, which were then detected by three different CCDs placed directly against the surfaces of the dichroric (or trichroric) beam splitter prism. However, while a dichroric or trichroric beam splitter prism would typically include low-pass and/or high-pass filter coatings at the interfaces between the prisms to separate the incident light into different wavelengths, a beam splitter system according to embodiments of the present disclosure is designed such each of its output beams 20 (the light reflected and transmitted at each interface) has substantially the same distribution of energy across the spectrum as the incident light 10.

FIG. 4A is a schematic depiction of an optical system including a prism beam splitter system 170 according to one embodiment of the present disclosure. In the embodiment shown in FIG. 4A, the beam splitter system 170 includes a first prism 171, a second prism 172, and a third prism 173. A first beam splitter 174 is located at an interface between the first prism 171 and the second prism 172, and a second beam splitter 175 is located at an interface between the second prism 172 and the third prism 173. The first beam splitter 174 and the second beam splitter 175 may include, for example non-polarizing beam splitter coatings.

An input beam 10 encounters the first beam splitter 174 and is partially reflected to produce first output beam 21 and partially transmitted to produce intermediate beam 24. The intermediate beam 24 is partially reflected by the second beam splitter 175 to produce second output beam 22 and partially transmitted to produce third output beam 23. As noted above, the first beam splitter 174 may be designed such that it reflects ⅓ of the input beam 10 and transmits ⅔ of the input beam, and the second beam splitter 175 may be designed such that it reflects ½ of the intermediate beam 24 and transmits ½ of the intermediate beam, such that the first output beam 21, the second output beam 22, and the third output beam 23 have substantially the same amount of light.

In the embodiment shown in FIG. 4A, the sensors 300, stacked with polarizing filters 200, are placed directly on the sides of the prisms of the beam splitter. In particular, the first polarizing filter 201 is stacked, with the first sensor 301, against one surface of the first prism 171, the second polarizing filter 202 is stacked, with the second sensor 302, against one surface of the second prism 172, and the third polarizing filter 203 is stacked, with the third sensor 303, against one surface of the third prism 173. This arrangement results in the sensors being registered with one another up to a 2-D translation (e.g., to account for shifting of each sensor on the surface of the prism where it is mounted), thereby reducing the effort of calibration.

In the embodiment shown in FIG. 4A, the first output beam 21 and the second output beam 22 each undergo two reflections before exiting the beam splitter system 100 and being detected by their respective sensors. Each reflection of the beam may introduce additional non-linearities or artifacts in the beam. Accordingly, some embodiments of the present disclosure relate to arrangements of a prism beam splitter to reduce the number of reflections.

FIG. 4B is a schematic depiction of an optical system including a prism beam splitter system according to one embodiment of the present disclosure. In the embodiment shown in FIG. 4B, the beam splitter system 180 includes a first prism 181, a second prism 182, and a third prism 183. A first beam splitter 184 is located at an interface between the first prism 181 and the second prism 182, and a second beam splitter 185 is located at an interface between the second prism 182 and the third prism 183. The first beam splitter 184 and the second beam splitter 185 may include, for example non-polarizing beam splitter coatings.

An input beam 10 encounters the first beam splitter 184 and is partially reflected to produce first output beam 21 and partially transmitted to produce intermediate beam 24. The intermediate beam 24 is partially reflected by the second beam splitter 185 to produce second output beam 22 and partially transmitted to produce third output beam 23. As noted above, the first beam splitter 184 may be designed such that it reflects ⅓ of the input beam 10 and transmits ⅔ of the input beam, and the second beam splitter 185 may be designed such that it reflects ½ of the intermediate beam 24 and transmits ½ of the intermediate beam, such that the first output beam 21, the second output beam 22, and the third output beam 23 have substantially the same amount of light.

Like the embodiment shown in FIG. 4A, in the embodiment of FIG. 4B, the sensors 300, stacked with polarizing filters 200, are placed directly on the sides of the prisms of the beam splitter. In particular, the first polarizing filter 201 is stacked, with the first sensor 301, against one surface of the first prism 181, the second polarizing filter 202 is stacked, with the second sensor 302, against one surface of the second prism 182, and the third polarizing filter 203 is stacked, with the third sensor 303, against one surface of the third prism 183. In the arrangement shown in FIG. 4B, the first output beam 21 and the second output beam 22 each undergo only one reflection, at the first and second beam splitters 184 and 185.

As described above, beam splitter systems 100 according to various embodiments of the present disclosure may introduce various artifacts and non-linearities such as wavelength dependence, polarization direction dependence, and the like. To correct for these non-linearities, some aspects of embodiments of the present disclosure relate to systems and methods for calibrating the optical system so that the per-pixel polarization values after correlation across the sensors 300 (e.g., three different sensors) are accurate to sub-pixel levels. In some embodiments, the controller 400 performs the calibration across the sensors of the sensor system 300. In some embodiments, the controller 400 processes the images captured by the sensor system 300 in accordance with a known set of calibration parameters, thereby correcting for the artifacts and non-linearities introduced by the beam splitter system 100.

A non-polarizing cube beamsplitter uses a dielectric to preserve the polarization of light in both the transmitted and reflected optical paths. However, the degree and angle of polarization is modified in the reflected optical path. Some aspects of embodiments of the present disclosure relate to reversing the changes to the degree and angle of polarization (DOLP and AOLP) using the Fresnel equations that describe the behavior of polarization light upon reflection at a dielectric surface of known refractive index n and angle of incidence θ. In particular, a calibration process according to embodiments of the present disclosure corrects for these changes so that the per-pixel polarization cues from the multiple sensors (e.g., three sensors) are corrected and properly aligned.

In some embodiments, the controller 400 is configured to recover the unmodified polarization state (e.g., the polarization state of the input beam 10 prior to modification by the beam splitter system 100) using Fresnel algebra. The following exemplary analysis assumes that the objects of the scene are placed in air and no ferromagnetic materials are present, which also simplifies the Fresnel algebra. However, embodiments of the present disclosure are not limited thereto, and similar corrections may be made in other environmental contexts with a different medium having a different refractive index.

Assuming that a pencil of light radiation is incident on the surface of the dielectric medium, the reflection and refraction of that light at this dielectric surface is fully defined by the Fresnel equations, which can also be derived from Maxwell's equations. The reflected light has both a s-polarization component (or perpendicular to plane of incidence) and a p-polarization component (or parallel to the plane of incidence). These are defined by the Fresnel equations as follows:

$\begin{matrix} {\gamma_{s} = \frac{{\cos\theta_{i}} - {n\cos\theta_{t}}}{{\cos\theta_{i}} + {n\cos\theta_{t}}}} & (1) \\ {\gamma_{p} = \frac{{n\cos\theta_{i}} - {\cos\theta_{t}}}{{n\cos\theta_{i}} + {\cos\theta_{t}}}} & (2) \end{matrix}$

where the subscripts i and t refer to the incident and transmitted components. Thus, θ_(i) refers to the angle of incidence, while θ_(t) refers to the angle of transmission, and n refers to the refractive index of the dielectric. The incident and transmitted components are related by Snell's law as

$\begin{matrix} {{n_{i}\sin\theta_{i}} = {n_{t}\sin\theta_{t}}} & (3) \\ {{\sin\theta_{t}} = {\frac{1}{n}\sin\theta_{i}}} & (4) \end{matrix}$

where n_(t)=1 and n_(t)=n, based on the assumption that the objects imaged are in air.

Substituting Equation (4) into Equations (1) and (2) produces Equations (5) and (6):

$\begin{matrix} {\gamma_{s} = \frac{{\cos\theta} - {n\sqrt{1 - \left( {\frac{1}{n}\sin\theta} \right)^{2}}}}{{\cos\theta} + {n\sqrt{1 - \left( {\frac{1}{n}\sin\theta} \right)^{2}}}}} & (5) \\ {\gamma_{p} = \frac{\sqrt{1 - \left( {\frac{1}{n}\sin\theta} \right)^{2}} - {n\cos\theta}}{\sqrt{1 - \left( {\frac{1}{n}\sin\theta} \right)^{2}} + {n\cos\theta}}} & (6) \end{matrix}$

where the subscripts on the θ have been dropped.

Now considering the Fresnel signal power coefficients R_(s) and R_(p) in the case of the incident light being unpolarized:

R _(s)=γ_(s) ²  (7)

R _(p)=γ_(p) ²  (8)

Based on the Fresnel signal power coefficients, the degree of linear polarization (DOLP) associated with the reflection on the beamsplitter can be expressed as:

$\begin{matrix} {{DOLP} = \frac{R_{s} - R_{p}}{R_{s} + R_{p}}} & (9) \end{matrix}$

Assuming unpolarized incident light, the DOLP can be calculated by substituting Equations (7) and (8) into Equation (9) to yield Equation (10):

$\begin{matrix} {{DOLP} = \frac{2\sin^{2}\theta\cos\theta\sqrt{n^{2} - {\sin^{2}\theta}}}{n^{2} - {\sin^{2}\theta} - {n^{2}\sin^{2}\theta} + {2\sin^{4}\theta}}} & (10) \end{matrix}$

However, in the context of a camera or imager that includes a beam splitter system 100, the DOLP of polarized incident light is modified by the beam splitter system 100 and therefore can be computed based on modified Fresnel signal power coefficients

and

=γ_(s) R _(s)  (11)

=γ_(p) R _(p)  (12)

where the modified

can therefore be written as a function ƒ of n, θ, γ_(s), γ_(p):

$\begin{matrix} {= {\frac{{\gamma_{s}R_{s}} - {\gamma_{p}R_{p}}}{{\gamma_{s}R_{s}} + {\gamma_{p}R_{p}}}\overset{\Delta}{=}{f\left( {n,\theta,\gamma_{s},\gamma_{p}} \right)}}} & (13) \end{matrix}$

The intensity of an image point corresponding to a point in 3-D space as captured by a camera with a polarizing filter at a certain angle is modulated by the angle of the polarizing filter. As the angle of the polarizing filter is varied, the intensity of that point varies sinusoidally and fluctuates between a maximum and minimum intensity value (I_(max) and I_(min)), which provide good estimates of R_(s) and R_(p) in the general case of a pixel behind a polarizing filter. To estimate the coefficients γ_(s) and γ_(p), in the specific case of a glass beam splitter, these can be written as:

γ_(s) =R _(s)(θ=45°,n=1.5)=0.0920  (14)

γ_(p) =R _(p)(θ=45°,n=1.5)=0.0085  (15)

As shown above, is possible to calculate the DOLP as well as the modified

by estimating the reflection coefficients, as described above, or, in some embodiments, by calculating DOLP to estimate

.

A similar Fresnel algebra can be used to estimate the modified angle of linear polarization

using the

and

modified Fresnel coefficients discussed above, which are calculated based on the coefficients γ_(s) and γ_(p) in Equations (14) and (15).

Accordingly, some embodiments of the present disclosure relate to the use of an estimation of the coefficients γ_(s) and γ_(p), as was used for DOLP, above, to compute the modified

and the modified

.

Without limitation thereto, one exemplary approach to computing calibration parameters identifies the perturbation to the phase of the transmitted radiance sinusoid (TRS) caused by the reflection off the beamsplitter. Once the perturbation is obtained, the system identifies an additive correction to the phase estimate to obtain the modified

, where the additive correction is used to correct for modifications to the angle of linear polarization caused by the beamsplitter. The perturbation coefficients γ_(s) and γ_(p) depend on refractive index n and angle of the beamsplitter θ, which are both known a priori, and therefore the values of the perturbation coefficients can be computed using Equations (5) and (6), above.

In some embodiments, the AOLP and DOLP signals on the reflected beams are recovered through acquiring empirical data from a carefully calibrated hardware setup.

FIG. 5 is one example of a cube beam splitter 190 set up in a controlled laboratory environment with a dielectric coating on the reflecting or beam splitting surface 193 between first and second prisms 191 and 192 for acquiring empirical data according to one embodiment of the present disclosure.

Incident polarized light 19 input into one end of the beam splitter is both partially reflected 19-1 and partially transmitted 19-2 by the beam splitting surface 193 and detected by two image sensors sensor_(r) 305 which detects the reflected beam 19-1 and sensors 306 which detects the transmitted beam 19-2. The two image sensors are calibrated for their camera intrinsic parameters, aligned for centration, corrected for distortion, and corrected for focal length differences.

Once calibrated (in the sense of determining camera intrinsic parameters), multiple images of different objects are captured and for each pair of images the intensity profiles are cross-correlated. The intrinsic differences in polarizations in the detected images can then be attributed to the modifications in the polarization state that were caused by reflection at the dielectric coating of the beamsplitter 193. By capturing multiple such image pairs of the reflected beams 19-1 and transmitted beams 19-2 for various objects and various angles of the polarizing filter, this calibration process generates a training dataset. The training dataset is then used to train a suitable statistical model or classifier (e.g., machine learning model) to estimate or predict a corrected polarization state (e.g., the polarization state of the incident polarized light 19) based on the polarization state of the reflected beam 19-1.

In addition to computing calibration parameters to correct for modifications in the polarization state of reflected beams, some aspects of embodiments of the present disclosure further relate to computing calibration parameters to correct for wavelength dependence of reflected versus transmitted light. As noted above, different coatings used at the beamsplitter may split an input beam (e.g., reflect or transmit the incident light) in a manner that is non-linear with respect to the wavelength of the incident light. A similar calibration process measures transmission and reflection curves over a range of wavelengths (e.g., over the visible spectrum), or where these transmission and reflection curves may be available from the manufacturer of the coating and/or beamsplitter. For example, a transmission curve may represent the percentage of light that is transmitted through the beamsplitter as a function of wavelength, and a reflection curve may represent the percentage of light that is reflected by the beamsplitter as a function of wavelength. As such, in some embodiments, the controller further corrects for non-linearities in the transmitted and reflected output beams 20 detected by the sensors 300 in accordance with the wavelength-dependent transmission and reflection curves associated with the beamsplitter.

FIG. 6 is a flowchart depicting a method 600 of computing corrected output images based on received input images and calibration parameters of a beam splitter according to one embodiment of the present disclosure. In some embodiments of the present disclosure, the method 600 is implemented by a processing circuit such as the controller 400. In operation 610, the processing circuit receives images captured from output beams (e.g., filtered output beams 30) that were previously split by a beamsplitter. In operation 630, the processing circuit computes output images based on the received images and calibration parameters associated with the beamsplitter. The output images may include, for example, polarization signatures (e.g., Stokes vectors, AOLP, DOLP, etc.), corrected images (e.g., color-corrected images to account for non-linearities in the wavelength-dependent transmission and reflection of light by the beamsplitter), corrected polarization raw frames (e.g., to correct for polarization imparted to reflected beams by the beamsplitter), and the like, as discussed above. In some embodiments, the processing circuit further applies camera parameters associated with the received images (e.g., intrinsic parameters associated with the sensors of the sensor system 300) to correlate the polarized intensity images and to compute per-pixel polarization values across the correlated polarized intensity images.

Accordingly, aspects of embodiments of the present disclosure relate to generating calibration parameters (e.g., a machine learning model) associated with a particular design of a beam splitter system 100 to correct for non-linearities and modifications applied by the beam splitter system 100 to the polarization states of input beams. Approaches in accordance with embodiments of the present disclosure use beam splitters to split the incoming light in a way that preserves the per-pixel polarization cues for the different polarization filters on each sensor through a combination of selection of the coating dielectric, positioning of the beamsplitter (e.g., prism beamsplitter), the sensors, and lenses relative to each other. Any changes to the polarization cues that remain after the system design are addressed through a subsequent one-time calibration step of the system that reverses any polarization changes and intensity variations in the different optical paths. The resulting images are pixel-aligned, and optical systems according to embodiments of the present disclosure can be easily customized to the demands of the applications in terms of the resolution and other parameters and furthermore can be made very cost effective.

While aspects of embodiments of the present disclosure are described in the context of beamsplitters, the approaches described herein apply more generally to correcting for multipath interference. Specifically, a beamsplitter introduces multipath interference. As discussed above, with knowledge of the surface angle and reflection it is possible to estimate and correct for multipath interference. This does not only apply to beamsplitters in the optical path, but also other reflections in the optical path, such as multipath caused by looking through a car windshield having known material properties and oriented with a surface at a known angle. Embodiments of the present disclosure therefore enable the teasing out multipath interference to polarimetric parameters on a variety of scene objects whose material and angular properties are known.

While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof. 

What is claimed is:
 1. An optical system comprising: a beam splitter system configured to split an input beam into a plurality of output beams comprising a first output beam, a second output beam, and a third output beam; a first polarizing filter having a first polarization angle and configured to filter the first output beam to produce a first filtered output beam; a second polarizing filter having a second angle of polarization and configured to filter the second output beam to produce a second filtered output beam; and a third polarizing filter having a third angle of polarization and configured to filter the third output beam to produce a third filtered output beam, the first, second, and third angles of polarization being different from one another.
 2. The optical system of claim 1, further comprising a sensor system configured to detect a plurality of polarized intensity images comprising: a first polarized intensity image of the first filtered output beam; a second polarized intensity image of the second filtered output beam; and a third polarized intensity image of the third filtered output beam.
 3. The optical system of claim 2, further comprising a processing circuit and memory, the memory storing instructions that, when executed by the processing circuit, cause the processing circuit to: receive the first polarized intensity image, the second polarized intensity image, and the third polarized intensity image; and compute a polarization signature based on the first polarized intensity image, the second polarized intensity image, the third polarized intensity image, and a plurality of calibration parameters associated with the beam splitter system.
 4. The optical system of claim 3, wherein the calibration parameters are computed based on computing modified Fresnel signal power coefficients in accordance with an index of refraction of the beam splitter system and an angle of the beam splitter system with respect to the input beam.
 5. The optical system of claim 4, wherein the calibration parameters comprise a modified degree of linear polarization and a modified angle of linear polarization.
 6. The optical system of claim 3, wherein the calibration parameters comprise a statistical model of the beam splitter system, the statistical model being trained on a dataset collected from images captured through the beam splitter system.
 7. The optical system of claim 6, wherein the dataset comprises first images corresponding to images captured from reflections of polarized light off a beam splitter of the beam splitter system and second images corresponding to images captured from transmissions of polarized light through the beam splitter of the beam splitter system.
 8. The optical system of claim 3, wherein the calibration parameters comprise wavelength-dependent transmission and reflection curves.
 9. The optical system of claim 3, wherein the processing circuit is configured to correlate the polarized intensity images and to compute per-pixel polarization values across the correlated polarized intensity images.
 10. The optical system of claim 1, wherein the plurality of output beams further comprises a fourth output beam.
 11. The optical system of claim 10, further comprising a fourth polarizing filter having a fourth angle of polarization and configured to filter the fourth output beam to produce a fourth filtered output beam, the fourth angle of polarization being different from the first, second, and third angles of polarization.
 12. The optical system of claim 10, further comprising: a low pass filter configured to filter the fourth output beam to produce a fourth filtered output beam; and a sensor configured to detect an infrared intensity image of the fourth filtered output beam.
 13. The optical system of claim 10, further comprising: a high pass filter configured to filter the fourth output beam to produce a fourth filtered output beam; and a sensor configured to detect an ultraviolet intensity image of the fourth filtered output beam.
 14. A method for computing a polarization signature, the method comprising: receiving, by a processor: a first polarized intensity image representing the intensity of a first output beam after passing through a first polarizing filter at a first polarization angle; a second polarized intensity image representing the intensity of a second output beam after passing through a second polarizing filter at a second polarization angle; and a third polarized intensity image representing the intensity of a third output beam after passing through a third polarizing filter at a third polarization angle, the first output beam, the second output beam, and the third output beam being split from an input beam by a beam splitter system; and computing, by the processor, a polarization signature based on the first polarized intensity image, the second polarized intensity image, the third polarized intensity image, and a plurality of calibration parameters associated with the beam splitter system.
 15. The method of claim 14, wherein the calibration parameters are computed based on computing modified Fresnel signal power coefficients in accordance with an index of refraction of the beam splitter system and an angle of the beam splitter system with respect to the input beam.
 16. The method of claim 15, wherein the calibration parameters comprise a modified degree of linear polarization and a modified angle of linear polarization.
 17. The method of claim 14, wherein the calibration parameters comprise a statistical model of the beam splitter system, the statistical model being trained on a dataset collected from images captured through the beam splitter system.
 18. The method of claim 17, wherein the dataset comprises first images corresponding to images captured from reflections of polarized light off a beam splitter of the beam splitter system and second images corresponding to images captured from transmissions of polarized light through the beam splitter of the beam splitter system.
 19. The method of claim 14, wherein the calibration parameters comprise wavelength-dependent transmission and reflection curves.
 20. The method of claim 14, further comprising correlating the polarized intensity images and computing per-pixel polarization values across the correlated polarized intensity images. 